The present invention is generally related to cellular telephone systems, and more particularly to an improved analog cellular telephone system capable of transmitting packet data.
Although the primary focus of cellular radiotelephone service has been on providing voice communications, there has been a rapid growth in demand for data services. Also, because cellular telephone systems provide performance characteristics of the land line telephone system and interconnect with it, subscribers expect land telephone features from the cellular telephone system. Thus, early solutions to demand for data have included use of a conventional modem to convert data to voiceband frequencies before transmitting the data from the subscriber unit (SU) over the cellular air interface, much like data is converted to voiceband frequencies before it is sent over land lines. However, the bandwidth provided for voice in analog cellular is 300 to 3000 Hz, which is narrower than that for land lines. Further, FM techniques such as companding and preemphasis modify a data signal in ways that modems built for land lines do not have to contend with, and problems arising from rapid multipath fading and handoffs further complicate cellular data transmission. The use of land line modems has thus proven unreliable except under favorable radio conditions.
As a result, refinements have been made, such as use of a specialized modem with error correction techniques suitable for a cellular channel, or use of an intermediate radio channel format for transmission over the air interface to a voice transceiver at a base station (BS), from where the data is reconverted and transmitted via a conventional modem to a public switched telephone network (PSTN). An example of this latter system may be found in U.S. Pat. No. 4,654,867, Labedz et al., assigned to the assignee of the present invention. However, these approaches have a limited bandwidth, and a need remains for cellular data systems that can deliver still greater data rates with high reliability to handle the increasing demand for data services over cellular channels.
More recently, packet-switching techniques, similar to those used in computer communications, have been developed for application in cellular radiotelephone environments. Unlike the earlier circuit-switched systems, these systems format the user data into data packets and rely on packet-switching to route the data packets to the end system (ES) data processor. One such approach is Cellular Digital Packet Data (CDPD), which is generally described in the CDPD System Specification Release 1.0 (Jul. 19, 1993); an early example of such may be found in U.S. Pat. No. 4,887,265, Felix, assigned to the assignee of the present invention. In these systems the channel is occupied by a user only for the duration of each packet transmission, rather than leaving the channel connection in place until a user disconnect. The main advantage of a CDPD system is channel conservation, allowing multiple users to send and receive short, bursty packet transmissions over the same channel, thus minimizing the number of channels needed for data services. CDPD may be implemented either using a dedicated radio channel (such as disclosed in the above U.S. Pat. No. 4,887,265), or by "borrowing" unused portions of a voice channel. In either case, all such prior art packet data systems have relied upon packet-switching for routing the packet data to the ES-data processor from the SU. But, although CDPD does achieve some channel conservation, it also limits the bandwidth available for the transmission of the user data. Such a limited bandwidth may not adversely affect smaller or slower applications such as E-Mail retrieval, but it is a problem when longer applications, such as file transfers or facsimile transmissions, are needed.
Finally, digital cellular systems have been proposed for providing the needed increase in bandwidth for data transmission, particularly those that are being designed to interface with ISDN like GSM (or Global System for Mobile communications). However, in countries like the U.S. digital cellular systems will most likely be phased-in so that current analog systems like AMPS (Advanced Mobile Phone Service) may continue to support existing subscribers for some time to come. There thus remains a need for a method and device for reliable and faster data transmission over analog cellular systems.